Since the early days of rotary drilling it has been desirable to know what is happening in the downhole location to drilling assemblies, and to the course of the well bore, before running surveys and tripping the drill string. There have been many efforts to contrive apparatus to send information upward through the drilling fluid flowing in the drill string bore. U.S. Pat. No. 3,065,416 issued in November, 1962, was an early use of the drilling fluid to power the signalling apparatus. That invention was a servo amplified, mud breathing, device used to determine the speed of a turbodrill while it was operating downhole. That apparatus, only slightly modified, is still in use for that purpose. Its repetition rate is too slow to satisfy the demand for higher data rates needed today.
Mud breathing apparatus depend upon some degree of mud conditioning not always present and reliable until recent years. In the intervening years, efforts were made to eliminate mud breathing for power to manipulate the signal valve. Notable among those efforts were apparatus using mud driven turbines driving generators to provide electric power to operate signal valves. Except for the signal valve elements, these apparatus were sealed and operated in an oil filled enclosure. Those systems are complex and costly to build and operate. If the installed systems fail downhole the drill string has to be tripped to accomplish repair and replacement.
More recent trends have been to provide apparatus that can be lowered, and recovered, through the drill string bore. Failures of the apparatus can then be addressed without tripping the drill string. Such apparatus are not of sufficient diameter to permit the use of mud driven electric generators and have to depend upon batteries carried by the general enclosure, usually referred to as a shuttle package. Available batteries will not last long enough if they have to power the signal valve. Interest was again directed to the servo valve controlled, mud breathing, signal valve operating systems. Time and effort, and better mud conditioning processes, have brought more reliability to those contrivances. With greater reliability, and their inherent simplicity, mud breathers are again being used even when installed rather than being shuttle packaged.
The U.S. Pat. No. 4,724,498 issued May, 1988, represents a more recent design of mud breathing, servo controlled, systems.
Two paramount problems have to be recognized in mud breathing signal valve actuators. Erosion by abrasive, high velocity, drilling mud tends to degrade all machine parts exposed to the effluent from the signal valve, and fine particle silting tends to paralyze moving parts exposed to mud in quiescent regions of the machinery.